Linking bone development on the caudal aspect of the distal phalanx with age and lameness history in pasture-based dairy cows: an <i>ex-vivo</i> digital photograph-based assessment

AIMS: To evaluate whether landmarks used to measure digital phalanx bone development using micro-CT could be used to measure bone development in digital photographs, and the association in pasture-based cows, between bone development, age and lameness history. METHODS: 160 distal phalanges (from the hind feet of 40 cows) were collected post-mortem, photographed, and measured at four sites (A-D) near the flexor tuberosity. Associations between age and lameness history, and the average sum of A-D for the two lateral and two medial claws within a cow (BD-Ave) and the highest sum of A-D for an individual claw (BD-Max) were modelled. RESULTS: Median age of enrolled cows was 8 (min 3, max 12) years and 34/40 had a history of lameness in the lactation before culling. Only minor modifications to the measurement landmarks were needed to account for the differences between micro-CT scans and digital photographs. Age was associated with BD-Max and BD-Ave: a 1-year increase in age increased geometric mean BD-Max by 1.12 (95% CI = 1.03-1.21) times and geometric mean BD-Ave by 1.09 (95% CI = 1.04-1.14) times. Prior lameness was compatible with an increase and decrease in geometric mean BD-Max and BD-Ave. Compared to cows with no history of lameness in the previous lactation, for previously lame cows, BD-Max was 1.61 (95% CI = 0.88-2.95) and BD-Ave, 1.25 (95% CI = 0.89-1.78) times greater, equivalent to a 5- and 3-year age effect, respectively. Most of the measurement differences between claws were in D (abaxial caudal bone development), as 136/160 claws recorded zero for A, B and C but all claws had values > 0 mm for D. In all claws where only D was > 0 mm, bone development was smooth. In these claws, an additional year in age increased D measurements by 1.09 (95% CI = 1.05-1.11) times - similar to the effect on BD-Max, while the effect on the sum of the remaining measurements was compatible with both a moderate increase or decrease. These observations indicate the principal effect of age was on measurement D. CONCLUSIONS: samples. Our data suggest lameness and age may affect bone development in pasture-based cows. However, these are preliminary data; further research is needed comparing CT and photography measurements, and to better understand the association between age, lameness and bone development in pasture-based dairy cows.